Rumb made all good points. For me I would just add to drain and clean the tank first. Normally the fluid is extremely dirty unless you had a fastidious owner. The fluid should be clear, at least golden in colour. What you will most likely find is a colour that looks like Coca-Cola! To drain the tank you need to remove it from the mounts (you can access the bottom mounts from the driver LHS fender well, much easier). You would also need to remove the pressure regulator from the top of the tank to do this. As Robert has stated these fittings can be hella tight after all these years. Localized heat with propane torch for sure, and you need to get some good flare/line wrenches otherwise you will round off the fittings for sure. They might even be too tight and you will have to wrench them off with vice grips, then replace with metric bubble flare brake lines. Wash the tank out with solvent or similar (kerosene etc), then mount the tank and drop in some fresh "ZH-M" fluid, about 2.5 - 3 litres from memory. You also need to replace the filter under the pressure regulator, they are super cheap and very good to replace on a regular basis. The system needs clean fluid to run well. Thus I would advocate to make sure you run as clean as you can for the first start up.
Indeed these cars sink extremely low to the ground and then you cannot move them without huge effort. Wooden blocks to start with just to give you some clearance is a good tip.
On a well sorted system, it should rise with no revving of the engine, in about 30 sec to 1 minute tops. At first the rise is slow but towards the end it rises quickly. Not rising or very slow rising usually points to a weak pump, or faulty pressure regulator or both. Pumps are a known weak spot but they can be rebuilt. Do not run ATF in the system, it will eat your seals! Once you have cracked the lines, you need to also bleed your pump. with the system down, open the connection from the pump at the pressure regulator, then point the open end of the hose into the filler hole of the tank, and start up the engine. This will cycle fluid through and purge air from the pump. Then reconnect after about minute of purge and start up.
Also a word on safety. The system runs at extreme pressure of about 2500 psi, so keep that in mind. Safety goggles and gloves are a must. Hydro oil under such pressure can pierce your skin and this can end up being fatal so be very careful!
Another very good site for 6.9 owners is www.w116.org